In recent years, a chip antenna for GPS (Global Positioning System) or Bluetooth is built in a small-size portable terminal such as a mobile-phone. A chip antenna of this type is required to be small in size and to facilitate resonance frequency adjustment and impedance matching. This is because the resonance frequency and the input impedance of the chip antenna are affected by the structure of the printed circuit board, various electronic components mounted around the chip antenna, and the housing. Therefore, it is necessary to adjust the resonance frequency and the input impedance for each model.
Particularly, it is very important to facilitate the input impedance adjustment of an antenna for the following reason. When the input impedance does not match a feeder-side impedance, VSWR characteristics of the antenna deteriorate and the antenna cannot exhibit performance inherent in the antenna. To facilitate input impedance matching, Japanese Patent Application Laid-Open No. 11-340726 discloses an antenna device having the following structure. A U-shaped radiation conductor, a ground conductor, and a feeder-to-ground short-circuit conductor are formed on an upper surface of a substrate, a bottom surface thereof, and a side surface thereof, respectively. An inductance value of the feeder-to-ground short-circuit conductor is changed by adjusting a branching point of the feeder-to-ground short-circuit conductor, thereby adjusting an input impedance of the antenna.
Furthermore, Japanese Patent Application Laid-Open No. 2003-69331 discloses the following surface-mount antenna. A double-housing (inverted-U) feeder electrode is formed on a side surface to an upper surface of a substrate, a length of the feeder electrode is adjusted, thereby changing an inductance value and matching an input impedance to a feeder-side impedance. In a case of this antenna, even when a capacitance between a radiation electrode and a feeder electrode increases because of use of a high permittivity material for the substrate, it is possible to increase the inductance of the feeder electrode, cancel an increase of the capacitance, and facilitate impedance matching.
However, with the conventional antenna structure described in the Japanese Patent Application Laid-Open No. 11-340726, the feeder-to-ground short-circuit conductor is formed in a wide range from the side surface to the upper surface of the substrate, which requires a sufficient area to form a conductor pattern. That is, there is a problem that the substrate needs to be high to some extent, and that it is difficult to make the substrate short.
Furthermore, with the conventional antenna structure described in the Japanese Patent Application Laid-Open No. 11-340726, the feeder-to-ground short-circuit conductor and the radiation conductor are capacitively coupled on the same plane or a plane orthogonal to the same plane. Intensity of capacitive coupling depends on a gap width. Therefore, when the gap width is small, a resonance frequency is low. When the gap width is large, the resonance frequency is low. Accordingly, when a low resonance frequency is to be obtained, a gap width should be set narrow. However, when the gap width is narrow, the antenna is made sensitive to a change in the resonance frequency and there is a problem that it becomes very difficult to adjust the resonance frequency. In addition, because of concentration of electric field on the narrow gap, there is also a problem that the bandwidth is made narrow.
A conventional antenna structure disclosed in Japanese Patent Application Laid-Open No. 2003-69331 has a similar problem to that disclosed in the Japanese Patent Application Laid-Open No. 11-340726. That is, a feeder electrode is formed on a side surface of a substrate and impedance matching is made by adjusting a length of the feeder electrode. Accordingly, it is necessary to secure an area necessary to form the feeder electrode on the side surface of the substrate, making it difficult to provide a short substrate.